Purpose :
Adaptive optics microstimulation can target single cone photoreceptors to measure threshold responses in vivo. Given the variation in cone dimensions, we used a modelling approach to estimate stimulus dimensions that optimally confine light delivery to a single cone.

Methods :
A waveguide model using 3D semi-vectorial finite difference beam propagation (FD-BPM) has been developed. The model propagates an electric field down a simplified cone photoreceptor and computes the field amplitude (Fig 1). Inner segments (IS) have myoid and ellipsoid compartments and are tapered parabolically from the IS to a cylindrical outer segment (OS). The model simulates 7 photoreceptors in a hexagonal pattern and estimates the power absorbed in each cone. Stimuli were positioned over the central cone, modelled with 543 nm light while varying pupil diameter, IS diameter, and stimulus size. The model did not include intraocular light scatter.Psychophysical validation of the model was performed using increment thresholds in 2 subjects measured at 3° eccentricity with 543 nm light, using stimuli deployed in the model. A subject’s threshold is assumed to be inversely proportional to the sum of the captured light across the 7 modelled cones. We compared the threshold ratio for beam sizes of 5.8 vs. 2.7 mm, while ensuring constant power, to assess whether perceptual performance matched model predictions.

Results :
The confinement of light absorption to single cones exceeds 95% in a regime where stimuli are about half the diameter of the cone IS and the pupil is larger than ~4 mm diameter, for 4 to 6 μm diameter cones (Fig. 2). The modelled dependence of light absorption on beam size was in agreement with psychophysical threshold ratios, for 3 stimulus conditions. The predicted vs measured (with SEM) ratios were: 0.84 vs. 0.74 ± 0.07 for 8.1 μm squares, 0.88 vs. 0.81 ± 0.09 for 9.9 μm circles and, 1.11 vs. 1.08 ± 0.12 for 4.5 μm squares.

Conclusions :
An FD-BPM model of light propagation for microstimuli in photoreceptors was developed and validated using increment thresholds. Optimal light coupling is a multidimensional function of stimulus size, IS size, and large pupil size under adaptive optics stimulus conditions.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.